Models for an “Objective” Assessment of Treatment Effects Based on Item Response Data

Charles Spearman’s contributions both to the quantitative methodology and to the theory of intelligence are part of the store of knowledge of any undergraduate in psychology. Most basic formulae of “classical test theory” are found already in his early publications (Spearman, 1904a, 1904b, 1907, 1910, 1913). In his seminal paper “General Intelligence,” objectively determined and measured (Spearman, 1904b), Spearman started out from reviewing the work of early test psychologists, many of whom had reported considerable correlations between various kinds of intellectual abilities, and concluded that there must exist some common element in all of them, some sort of a general intellectual ability, the “g-factor.”